To understand the origins of ISO, we have to take a short journey back through the mists of time to the days of film photography and go from there...
ISO - The International Organization for Standardisation
The acronym ISO itself is a reference to the International Organisation for Standardisation.
In relation to photography the ISO value of film was, and still is, determined by a scale of a film's sensitivity to light. The lower the numerical value the more sensitive to light.
Previously, film sensitivity was also measured a similar way by another organisation, the ASA or American Standards Association.
This has been superseded by ISO in modern times, but the measurement itself and the scale remain effectively the same.
However this organisation does far more than define camera sensitivities, it promotes universal standards for measurements of all different types, on an international level.
Fun fact: Instead of calling themselves the IOS, the title “ISO” is in reference to isos, (ίσος) which means “equal”.
Film ISO vs. Digital ISO
For film photography, ISO or ASA (American Standards Associations) speed refers to the film speed of the film roll. This is calculated by the films sensitivity to light; i.e. the more sensitive the film is to light, the less time it needs travel across the open shutter. Typically, when you are shooting outdoors on a sunny day, you would use an ISO100 or ISO200 film. If you’re shooting indoors, you would probably switch to an ISO800 film or faster. What’s challenging, of course, is if you have to go from an outdoor to indoor location quickly or in rapidly changing light conditions, because that usually means that you would either have to change the roll of film or compensate with your aperture or shutter speed.
The great thing about digital photography is that you can change your ISO speed on the fly, making it easy to transition between exterior and interior shots. On top of that, you can actually view on the LCD screen how your image looks in that particular ISO.
How ISO Affects Image Quality and Noise
In both film and digital photography, the higher the ISO, the more grain you’ll have in the image. In digital photography, noise is the by-product of the increased electric charge needed to make the sensor more sensitive to light and looks like speckles on the image. The consequence of more noise, however, is a rougher-looking image and a decrease in image quality.
Luminance Noise vs Chroma Noise
There are two types of noise - Luminance Noise, and Chroma (colour) Noise.
Luminance noise; retains much of the original colour because this type of noise only affects the brightness of the pixels.
Chroma noise; can look like coloured speckles or grain, and is largely unattractive. This is because the noise is affecting the colour of the pixels rather than just the brightness of the pixels. Fortunately, most post-processing software can do a good job in minimizing chroma noise.
Different cameras have different thresholds on when this noise starts to degrade the image quality. This is known as the signal-to-noise ratio. There are several factors that determine signal-to-noise ratio. Aside from the processor of the camera, the megapixel count and the size of the sensor play a role in how well a camera can minimize noise.
How a Camera’s Megapixels and Sensor Size Affects ISO
The size of the sensor and the amount of pixels on that sensor directly affects the potential amount of noise that can occur when you are shooting at higher ISOs. Imagine that a sensor is like a swimming pool and the pixels are the amount of beach balls that can float in that pool. If you only have 100 balls, you can fit larger size balls in the pool. If you want to fit 1,000 balls, you would either have to have a larger swimming pool or use smaller balls. That is essentially the same relationship with pixel count and sensor size.
A sensor is made up of millions of tiny light-gathering receptors called pixels. One megapixel (MP) consists of one million pixels. If you have two same size sensors and one has 12MP and the other has 24MP, the 12MP sensor can have larger pixels than the 24MP sensor.
The larger the pixel size, the better that pixel is in gathering light, just like the larger the beach ball, the more air it can hold. If you want to increase the number of pixels from 12MP to 24MP without decreasing the pixel size, then you would have to increase the physical sensor size.
This is like having a larger swimming pool to hold more beach balls without decreasing the size of the balls. The size of the pixel in relation to the sensor size